The present invention relates to a hydraulic drive system for a construction machine, such as a hydraulic excavator, in which a delivery pressure of a hydraulic pump is held higher than a maximum load pressure of a plurality of actuators by a target differential pressure under load sensing control, and differential pressures across a plurality of directional control valves are controlled by respective associated pressure compensating valves. More particularly, the present invention relates to a hydraulic drive system including a safety device to lock an actuator when it is in an inoperative condition while an engine is being driven, thereby preventing a malfunction.
A construction machine, such as a hydraulic excavator, includes a safety device for making an actuator immobile even with a control lever manipulated, thereby preventing the machine from malfunctioning, when an operator is not boarded on the machine while an engine is being driven, or when an operator is boarded on the machine, but no work is carried out. When a directional control valve has a pilot-operated spool, a safety device is generally constructed such that a pilot lock switching valve is provided between a pilot pump and a pilot valve of a control lever device, and by shifting the pilot lock switching valve, supply of a hydraulic fluid to the pilot valve of the control lever device is cut off to make the directional control valve locked. One example of that type of the pilot lock switching valve is disclosed in, e.g., Japanese Patent No. 2567720.
Also, as a hydraulic pump control system, there is known the so-called load sensing system (hereinafter referred to also as the xe2x80x9cLS systemxe2x80x9d) in which a delivery pressure of a hydraulic pump is held higher than a maximum load pressure of a plurality of actuators by a target differential pressure. Usually, in the LS system, differential pressures across a plurality of directional control valves are controlled by respective associated pressure compensating valves so that a hydraulic fluid can be supplied at a ratio depending on opening areas of the directional control valves regardless of the magnitudes of load pressures during the combined operation in which a plurality of actuators are driven at the same time. Hydraulic drive systems including LS systems are disclosed in, e.g., JP,A 60-11706 and JP,A 10-196604. In such a hydraulic drive system including an LS system, when a directional control valve has a pilot-operated spool, it is also general that a pilot lock switching valve similar to the above-mentioned one is provided as a safety device.
As described above, a conventional safety device (pilot lock switching valve) for a hydraulic drive system is based on an assumption of a directional control valve being pilot-shifted, and is constructed so as to cut off supply of a hydraulic fluid to a pilot valve of a control lever device, whereby the directional control valve is locked to make an associated actuator locked. However, the directional control valve is not limited to the pilot-shifted one, but may be mechanically shifted by transmitting a motion of a control lever directly to a spool for operating it.
For example, in many of small-sized hydraulic excavators having small swing bodies, such as mini-shovels, a directional control valve for travel is mechanically shifted. Also, in hydraulic excavators, a bucket is usually mounted as a front attachment of a front operating mechanism. With increasing versatility of work, however, it is now general that the bucket is replaceable by another front attachment such as a crusher. In many of such cases, a directional control valve associated with a front attachment other than the bucket is also designed as a mechanically shifted valve. Further, the directional control valve associated with the front attachment other than the bucket is either assembled in a valve unit beforehand or retrofitted to the valve unit.
Thus, when the hydraulic drive system includes a mechanically shifted directional control valve, or when a mechanically operated directional control valve is retrofitted to the hydraulic drive system, the conventional safety device cannot lock the directional control valve and hence cannot make the associated actuator locked.
Another conceivable solution for locking a mechanically shifted directional control valve is to fix a control lever mechanically, but this solution would entail a complicated mechanism.
An object of the present invention is to provide a hydraulic drive system including pressure compensating valves controlled by an LS system, in which an actuator can be locked with a simple construction and can be prevented from malfunctioning in an inoperative condition while an engine is being driven, even when the hydraulic drive system includes a mechanically shifted directional control valve, or even when a mechanically shifted directional control valve is retrofitted to the hydraulic drive system.
(1) To achieve the above object, according to the present invention, there is provided a hydraulic drive system comprising a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of directional control valves, and pump control means for performing load sensing control to hold a delivery pressure of the hydraulic pump higher than a maximum load pressure of the plurality of actuators by a target differential pressure, the plurality of pressure compensating valves including a first pressure compensating valve provided in association with a particular one of the plurality of directional control valves and a second pressure compensating valve provided in association with the other directional control valve than the particular one, wherein the hydraulic drive system further comprises a first lock switching valve having first and second shift positions and outputting a pressure of a hydraulic supply source when the first lock switching valve is shifted from the first position to the second position; and a first pressure receiving section provided at an end of the first pressure compensating valve on the si de acting in the closing direction, and connected to the output side of the first lock switching valve, the first pressure compensating valve being fully closed when the first lock switching valve is shifted to the second position and the pressure of the hydraulic supply source is introduced to the first pressure receiving section.
Thus, the first lock switching valve is provided, the first pressure receiving section is provided in the first pressure compensating valve to be connected to the output side of the first lock switching valve, and the pressure of the hydraulic supply source is introduced to the first pressure receiving section when the first lock switching valve is shift ed to the second position, thereby fully closing the first pressure compensating valve. With such an arrangement, even when the particular directional control valve is a mechanically shifted valve, the actuator associated with the particular directional control valve can be locked and hence prevented from malfunctioning in an inoperative condition while an engine is being driven. Also, since the first pressure receiving section can be provided by utilizing a pressure receiving section that is originally provided in an ordinary pressure compensating valve for a drain passage, the actuator can be locked with a simple construction. Moreover, since a main passage for supplying the hydraulic fluid to the actuator therethrough is cut off by the first pressure compensating valve, the actuator can be reliably locked.
Further, even when a mechanically shifted directional control valve for a front attachment is added to employ an additional attachment such as a crusher, an actuator for the attachment can be locked with a simple construction by introducing an output pressure of the first lock switching valve to a pressure receiving section of an associated pressure compensating valve.
(2) In the above (1), preferably, the particular directional control valve is a mechanically shifted valve, and the other directional control valve than the particular one is a pilot-shifted valve driven by a pilot control pressure.
(3) In the above (1) or (2), preferably, the hydraulic drive system further comprises a pilot hydraulic source; operating means connected to the pilot hydraulic source via a pilot line, generating the pilot control pressure based on a hydraulic pressure of the pilot hydraulic source, and including pilot valves for driving the other directional control valve than the particular one; a second lock switching valve disposed in the pilot line, having third and fourth shift positions, and cutting off the pilot line when the second lock switching valve is shifted from the third position to the fourth position, the second lock switching valve being operated by an operator; and interlock switching means for shifting the first lock switching valve from the first position to the second position in interlock with shifting of the second lock switching valve from the third position to the fourth position.
With those features, when the second lock switching valve is shifted from the third position to the fourth position, the pilot line is cut off and the operating means can no longer generate the pilot control pressure, whereby the actuator associated with the other directional control valve than the particular one can be locked. At the same time, the first lock switching valve is shifted from the first position to the second position in interlock with the shifting of the second lock switching valve. Therefore, the actuator associated with the particular directional control valve can be locked as mentioned in the above (1).
(4) In the above (3), preferable, the hydraulic drive system further comprises a second pressure receiving section provided at an end of the second pressure compensating valve on the side acting in the closing direction, and connected to the output side of the first lock switching valve.
With that feature, for the actuator associated with the other directional control valve than the particular one, dual lock functions of locking the actuator are provided by locking both the other directional control valve and the second pressure compensating valve. Therefore, that actuator can be more reliably locked.
(5) In the above (3), preferably, the interlock switching means includes a third pressure receiving section which is provided at an end of the first lock switching valve on the side acting to shift the first lock switching valve to the first position, and which is connected to the pilot line on the output side of the second lock switching valve.
With that feature, when the second lock switching valve is shifted to the fourth position, the first lock switching valve can be shifted to the second position.
(6) In the above (1) or (2), preferably, the hydraulic drive system further comprises a pilot hydraulic source; operating means connected to the pilot hydraulic source via a pilot line, generating the pilot control pressure based on a hydraulic pressure of the pilot hydraulic source, and including pilot valves for driving the other directional control valve than the particular one; a second lock switching valve disposed in the pilot line and having third, fourth and fifth shift positions, the second lock switching valve being operated by an operator; and a third pressure receiving section provided in the first lock switching valve and shifting the first lock switching valve from the second position to the first position when the pressure of the pilot hydraulic source is introduced to the third pressure receiving section, the second lock switching valve connecting the pilot line to both the pilot valves and the third pressure receiving section when the second lock switching valve is in the third position, cutting off the connection between the pilot line and both the pilot valves and the third pressure receiving section when the second lock. switching valve is in the fourth position, and cutting of the connection between the pilot line and the pilot valves and connecting the pilot line to the third pressure receiving section when the second lock switching valve is in the fifth position.
With those features, when the second lock switching valve is shifted from the third position to the fourth position, the connection between the pilot line and the pilot valves is cut off and the operating means can no longer generate the pilot control pressure. Therefore, the actuator associated with the other directional control valve than the particular one can be locked. At the same time, the connection between the pilot line and the third pressure receiving section of the first lock switching valve is cut off and the first lock switching valve is shifted from the first position to the second position in interlock with the shifting of the second lock switching valve. Therefore, the actuator associated with the particular directional control valve can be locked as mentioned in the above (1).
Further, when the second lock switching valve is shifted to the fifth position, the connection between the pilot line and the pilot valves is cut off, and hence the actuator associated with the other directional control valve than the particular one can be locked. On the other hand, since the pilot line is connected to the third pressure receiving section of the first lock switching valve, the first lock switching valve takes the first position and the pressure of the hydraulic supply source is no longer introduced to the first pressure receiving section of the first pressure compensating valve. Accordingly, the first pressure compensating valve is not fully closed and is capable of operating usually, whereby only the actuator associated with the particular directional control valve can be unlocked. In other words, it is possible to lock the actuator associated with the other directional control valve than the particular one, and to selectively unlock only the actuator associated with the particular directional control valve.
(7) In the above (6), preferably, the hydraulic drive system further comprises a second pressure receiving section provided at an end of the second pressure compensating valve on the, side acting in the closing direction, and connected to the, output side of the first lock switching valve.
With that feature, as mentioned in the above (4), for the actuator associated with the other directional control valve than the particular one, dual lock functions of locking the actuator are provided by locking both the other directional control valve and the second pressure compensating valve.
(8) In the above (1) or (2), preferably, the hydraulic drive system further comprises a pilot hydraulic source; operating means connected to the pilot hydraulic source via a pilot line, generating the pilot control pressure based on a hydraulic pressure of the pilot hydraulic source, and including pilot valves for driving the other directional control valve than the particular one; a second lock switching valve disposed in the pilot line, having third and fourth shift positions, and cutting off the pilot line when the second lock switching valve is shifted from the third position to the fourth position, the second lock switching valve being operated by an operator; and lock operating means enabling the first lock switching valve to be shifted between the first position and the second position when the second lock switching valve is in the fourth position.
With those features, when the second lock switching valve is shifted from the third position to the fourth position by the lock operating means, the pilot line is cut off and the operating means can no longer generate the pilot control pressure. Therefore, the actuator associated with the other directional control valve than the particular one can be locked. Also, by shifting the first lock valve from the first position to the second position at that time, the actuator associated with the particular directional control valve can be locked as mentioned in the above (1).
Further, when the first lock switching valve is shifted to the first position by the lock operating means in a condition of the second lock switching valve being in the fourth position, the pressure of the hydraulic supply source is no longer introduced to the first pressure receiving section of the first pressure compensating valve. Accordingly, the first pressure compensating valve is not fully closed and is capable of operating usually, whereby only the actuator associated with the particular directional control valve can be unlocked. In other words, it is possible to lock the actuator associated with the other directional control valve than the particular one, and to selectively unlock only the actuator associated with the particular directional control valve.
(9) In the above (8), preferably, the hydraulic drive system further comprises a third lock switching valve having sixth and seventh shift positions and outputting the pressure of the hydraulic supply source when the third lock switching valve is shifted from the sixth position to the seventh position; interlock switching means for shifting the third lock switching valve from the sixth position to the seventh position in interlock with shifting of the second lock switching valve from the third position to the fourth position; and a second pressure receiving section provided at an end of the second pressure compensating valve on the side acting in the closing direction, and connected to the output side of the third lock switching valve.
With that feature, as mentioned in the above (4), for the actuator associated with the other directional control valve than the particular one, dual lock functions of locking the actuator are provided by locking both the other directional control valve and the second pressure compensating valve.
(10) In the above (8), preferably, the first and second lock switching valves are mechanically shifted valves directly shifted by control levers, and the lock operating means includes the control levers.
(11) In the above (8), preferably, the first and second lock switching valves may be solenoid-shifted valves shifted by electrical signals. In this case, the lock operating means includes a controller for generating the electrical signals.